Recent years have seen a rapid increase in the number of experiments designed to assess the role of marine biodiversity on ecosystem processes. The diversity of study systems in which these questions have been addressed offers an opportunity to seek broadly applicable generalities, but this same diversity has also been an impediment to a simple synthesis due to different approaches, terminology and questions. In this talk, we present emerging generalities from our recent review of these studies, and discuss specific illustrative case studies. In studies of plant and sessile animal diversity, in general diversity per se often has weak effects on measures of productivity or biomass. However, over the longer term, the stability or consistency of production or other ecosystem function is often greater, or the coefficient of variation in function lower, in more diverse communities: this appears to be a relatively robust trend. Marine studies have led the field in investigating the effects of predator and consumer diversity on community regulation. Some generalities are emerging: diverse grazer communities are (1) generally more resistant to top-down control than depauperate ones, (2) use their own resources more completely and (3) often increase predator fitness by providing a balanced diet. In contrast the effects of predator diversity on top down control appears more varied because of the diverse ways in which predators interact with one another: intraguild predation, facilitation, and complementarity, and because of omnivory. We close with suggestions of areas in need of further attention in this rapidly moving field.